Specific, cloned DNA probes derived from Xenopus laevis embryonic polyA+RNA will be used to determine the mechanism by which a number of relatively abundant mRNA species accumulate in polysomes during early development. The exact sizes of these RNAs will be established in oocytes and their accumulation in polysomes following fertilization will be measured. We will determine whether the few polyA+RNA species whose concentrations increase during the first few hours after fertilization are the result of differential polyadenylation or require new synthesis. We will determine whether the accumulation of qualitatively new RNA in polysomes later in development is the result of the recruitment of pre-existing, non-polysomal molecules or is the result of increased rates of synthesis or increased stability. We will examine non-polysomal RNP and measure its ability to support protein synthesis in vitro in lysates derived from Xenopus oocytes and embryos. To understand the large accumulation of new mRNA in polysomes at the beginning of organogenesis we will examine the structures of genes which are very active at that time and contrast these structures with those of genes which are not active during this period.